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The Pannonian basin is in a Neogene extensional setting formed between the Carpathian and Dinaride thrust belts within the mega-suture zone of the African and European plates (Figure-1). According to thermo-tectonic models (Royden et al. 1983a, b; Royden 1988), the initial crustal thinning or rifting occurred in the Middle Miocene and the subsequent thermal subsidence or post-rift phase extended up to the present.

The sites of sedimentation in the syn-rift phase of the Pannonian basinformation were graben structures opened by different amounts of extension. The accommodation for sedimentation in the post-rift phase was controlled by rapid thermal subsidence when 1-6 km deep troughs and subbasins formed. The basin separated from the sea when uplift of the Dinarides and Carpathians occurred, and eventually, a brackish water lake formed. Extremely rapid deltaic sedimentation filled the basin, which prograded at rates of 30-100 km/Ma. The surrounding mountains served as sediment sources.

A significant unconformity was observed in the Pannonian basin which divides the basin fill strata into two major units. Correlations provided an age of about 6 Ma corresponding to the latest Miocene (Messinian) (Csato 1989; 1993). The age of the unconformity was determined based on seismic correlations of paleo-magnetic age data (Pogácsás et al. 1990). The event that formed this unconformity in the Pannonian basin was coeval with the salinity crisis in the Mediterranean (Hsü et al. 1977; Cita 1982) when the Mediterranean sea desiccated giving rise to massive evaporation.

Two major sedimentary systems were responsible for the infilling of the post-rift Pannonian basin: a delta which prograded from the northeast and another from the northwest (Figure-2). The Messinian relative lake level fall produced a widespread lowstand systems tract, largely along the northeastern paleo-margin of the basin. During the subsequent lake level rise the basin filled rapidly in Pliocene-Quaternary time. The northern portion of the lake was very narrow at 6 Ma, and the prograding shoreline shows evidence of intercalation produced by deltaic deposits from the two major transport directions.

Stratigraphic simulation of SEDPAK package (Kendall et al. 1991, 1993) on seismic section of Figure 2 revealed (Csato 1995) that the subsidence histories on opposite sides of the basin were very different (Figure-3). The northwest subbasin subsided continuously without major interruption as shown in the burial plot at location A, whereas the northeastern portion of the basin slowed down at 6 Ma. At location B, minor, or no subsidence is detectable for the time interval of 6-2.5 Ma. The lowstand systems tract of the northwestern subbasin is much thinner than that of the northeast and closely resembles a shelf margin systems tract. The simulation indicates that the Messinian lake level fall caused the formation of lowstand systems tracts, but differential subsidence was responsible for the different geometries seen on the two sides of the basin.

The cessation of subsidence for a period of time in the northeastern part of the Pannonian basin can be explained by basin inversion. Based on interpretations of Glennie and Boegner (1981), Cooper and Williams (1989), Doglioni (1990) and Bott (1992), inversion in rifts is usually driven by a change in orientation of stress field. The other cause of uplift in rift settings is the flexural rebounding after extension (Kusznir and Egan, 1990; Kusznir and Ziegler, 1992).

The seismic section of Figure 2 shows that the sediments derived from the northeast and northwest respectively, interfinger within the center of the section (mark IZ in Fig. 2). It is evident from the section, that sediment influx from the northwest became dominant after the Messinian event, and the northeastern sediment system remained subordinate.

It can be concluded that the Messinian event in the Pannonian basin was complex (Figure-4). A significant lake level fall occurred around 6 Ma in association with the desiccation of the Mediterranean. Probably independently from this base level fall, a tectonic reorganization affected the northeastern part of the basin causing the cessation of its subsidence. Simultaneously, the rest of the basin continued to subside at high rates. This areal change in subsidence distribution modified the flow directions of river drainage. Following the Messinian event, the major sediment transport in the basin was derived from the northwest. eustasy-related lake level fall, local transpressive tectonics and consequent change in sediment transport direction and intensity are proposed as the components of the complex event in the Pannonian basin occurred in the Messinian.

2. Research Problem

Subsurface data give evidence that a considerable lake level fall occurred in the isolated Pannonian basin coeval with the Messinian salinity crisis in the Mediterranean. Age dating is based on magnetostratigraphic correlations. Additionally, palynologic analysis on cores revealed dynoflagellate species typical in the Mediterranean at Messinian time (M. Sutone Szentai, personal communications). The Messinian event was so significant in the Pannonian basin, that the basin fill can be divided into two major sections along the Messinian unconformity.

The question arises: how could an isolated lacustrine basin experience a lake level change synchronously with a eustatic event? Although, fresh water fauna exclude marine connections, was the lake level controlled in some way (i.e. through rivers) by eustasy? The various stratigraphic architectural patterns that formed at the same time, suggest active tectonic activity. What could be the dynamic link between the Pannonian basin area and the Mediterranean?

3. Fluid Flow Modeling

This part of the project explores the compaction and fluid flow history in the Pannonian basin, with a special emphasis on the Mesinian event. The purpose of the project is to reveal the changes in fluid flow patterns as a consequence of the complex and rapid tectono-paleogeographic reorganization in the Messinian. The Messinian events caused partial uplift, erosion and simultaneous accelerated subsidence and sediment accumulation in other parts of the basin. The spatial distribution of deposition and compaction became largely variable. Part of the basin underwent subaerial emergence allowing significant meteoric water recharge. The gravitational flow may have caused a regional flush in the basin that affected the thermal flow and the temperature distribution in the sedimentary fill. The compressional stress that suddenly elevated parts of the basin, most likely produced overpressured compartments. The Messinian event reshaped the basin architecture, compartmentalized the sedimentation, deposition, compaction, fluid expulsion, fluid pressure and temperature development in the basin. Consequently, the project is expected to reveal the hydrocarbon geology implications of the Messinian Problem.

The enclosed figures demonstrate the potential of computer modeling in analyzing the evolution of temperature, pressure and fluid migration in response to changes in tectonics and sediment influx. Figure-5 summarizes the input data provided for the quantitative modeling. The output of lithology distribution is printed in Figure-6 that shows good match with the input information. Figure-7 and Figure-8 represent the fluid flow system prior to the Messinian event and at the onset of the compression, respectively.